In vertical cyclone furnaces, fuel may be injected tangentially in an upper half of the vertical cyclone furnace together with primary combustion air, and secondary combustion air may be tangentially injected in the same plane as that at which the primary combustion air is injected or higher in the primary combustion chamber. Ash is generally removed from a bottom area of the furnace by a rotating cooled ash scrapper, and waste gas is transferred through an aperture in a top of the vertical cyclone furnace to a secondary combustion chamber.
In, for example, U.S. Pat. No. 4,398,477, two combustion chambers are provided which consist of two cyclone furnaces arranged one above the other and connected through an opening with a reduced clearance, a so-called throat. The fuel, which consists of rice hulls, is blown together with the primary air in the lower vertical cyclone furnace, and the waste gas is then burned in the upper cyclone furnace during the introduction of additional combustion air through tangential nozzles. Through this arrangement an optimal incineration of the fuel is achieved, and the residual product in the form of ash can be removed from the bottom lower cyclone furnace by a cooled rotating ash scrapper.
In order to achieve optimal incineration of the fuel, the temperature in the lower furnace is on the order of 1200.degree. C. Such a high temperature is unfavorable in that, during combustion of biological fuels at this temperature, relatively large amounts of poisonous nitrogen oxides (NOx) are formed.
In WO90/05272, a sludge drying apparatus is proposed wherein, for example, sewage sludge is dried down to less than 10% water content in a rotating dryer, after which the dried sludge is used as fuel in a furnace which delivers the thermal energy necessary for the rotating dryer. However, it has proven to be almost impossible to incinerate the dried sludge in a normal cyclone furnace, with the reason being that the dried sludge and similar types of fuel vitrify to form a type of slag filled with porous pores which have an insulating effect, while, at the same time, the slag is highly viscous, thus rendering the removal of the slag impossible. Therefore, use is made in practice of other types of furnaces, for example, fluid-bed ovens, for the incineration of fuels which are aqueous or low energy content, such as, for example, dried biological sludge. Furnaces of such a type are suitable only for large amounts of fuel and require a long start-up time, and thus furnaces of this type are not suitable if they cannot be used in a continuous operation. Moreover, this type of furnace demands a comprehensive process regulation by specially-trained personnel.
Consequently, when it is necessary to dispose of refuse from smaller towns or urban areas, it is necessary to either use other methods of the disposal of the biological sludge or to transport the sludge to larger communal plants for incineration.